Apparatus for producing reduced iron and compact drying method applied to the apparatus

ABSTRACT

An apparatus for producing reduced iron, and a compact drying method for application to the apparatus are disclosed. The apparatus comprises a pelletizer or a briquetter for mixing and agglomerating coal as a reducing agent and iron ore as iron oxide to form compacts, a dryer for drying the compacts, a circular rotary hearth type reducing furnace for reducing the dried compacts in a high temperature atmosphere, a first heat exchanger for performing heat exchange between a hot off-gas discharged from the reducing furnace and combustion air to be supplied to the reducing furnace, and coolers for cooling the hot off-gas. A second heat exchanger for heating drying air is disposed on an exit side of the first heat exchanger. The drying air heated by the second heat exchanger is supplied to the dryer to dry the compacts with the drying air scant in moisture. Consequently, highly efficient, stable drying in the dryer can be performed, and high quality reduced iron can be produced stably.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus for producingreduced iron, which comprises mixing a reducing agent and iron oxide,agglomerating the mixture, drying the compacts (pellets, or briquettes)in a dryer, and reducing the dried compacts in a high temperatureatmosphere in a reducing furnace. This invention also relates to amethod for drying compacts, which method is applied to the apparatus.

[0003] 2. Description of the Related Art

[0004] To produce reduced iron, the first step is to mix an iron orepowder, a coal powder, a limestone powder, and a binder, and compressand agglomerate the mixture to form wet compacts called green compacts.Then, the wet compacts are dried to some degree to form dry compacts.The dry compacts are heated to a high temperature in a reducing furnace,where iron oxide in the iron ore is reduced with the coal (a reducingagent) to form reduced iron compacts.

[0005] An example of a conventional apparatus for producing reduced ironis explained by way of FIG. 6. At an upper portion of a circular rotaryhearth type reducing furnace 1, there are provided a device 2 forintroducing compacts (dry compacts), and an off-gas duct 3 fordischarging a hot off-gas, a residual form of a gas used in reduction.Inside the furnace 1, a discharger 4 is provided for discharging reducedcompacts P (reduced iron compacts). On a circumferential side wall ofthe furnace 1, a plurality of burners 5 are provided for generating areducing hot gas.

[0006] Powders of coal (a reducing agent), iron ore, etc. as rawmaterials are mixed with a binder, and the mixture is fed to apelletizer or a briquetter 6, where compacts (wet compacts) are formed.The resulting compacts are sent to a dryer 7, where the compacts aredried at about 120 to 150° C. to become dry compacts. The dry compactsare supplied to a rotary hearth of the reducing furnace 1 via theintroducing device 2.

[0007] In the reducing furnace 1, fuel and combustion air are fed to theburners 5, which generate a high temperature hot gas. The hot gas turnsin the direction of a dashed arrow, and during this motion, exerts areductive action on the compacts, an object to be treated, in a hightemperature atmosphere. A hot off-gas discharged through the off-gasduct 3 is primarily cooled by a primary cooler 8 of a water spray type,and then brought to a heat exchanger 9, where the off-gas exchanges heatwith the combustion air. Further, the off-gas is secondarily cooled by asecondary cooler 10 of a water spray type to about 300° C., for example.Then, the cooled off-gas is conveyed to the dryer 7 to dry the compacts.Then, the off-gas is passed through a dust collector 11, where it iscleaned, and then dissipated into the air.

[0008] When the rotary hearth inside the reducing furnace 1 makes nearlyone rotation in the direction of a solid arrow in FIG. 6, the reducedcompacts P are discharged from the screw type discharger 4. The compactsare delivered to a portable container 13 by a discharge chute 12, andthen transported to a subsequent step.

[0009] In the dryer 7 of the foregoing conventional apparatus forproducing reduced iron, the hot off-gas discharged from the reducingfurnace 1 and cooled by the water spray type primary cooler 8 andsecondary cooler 10 is used as a heat source for drying the compacts(wet compacts) at a temperature of about 120 to 150° C. That is, thecompacts are dried with the hot off-gas very rich in steam. Hence, on anunsteady occasion immediately after initiation of operation of theapparatus, moisture is condensed onto the surfaces of the wet compacts.As a result, sticking of the wet compacts to each other occurs,whereupon the wet compacts may lump, becoming large masses. In asituation such as that immediately after start of operation, theproperties, such as temperature and flow rate, of the hot off-gasdischarged from the reducing furnace 1 are not stable, so that drying inthe dryer 7 is unstable. This may cause the problem of wet compactlumping.

[0010] The wet compacts, which have been treated in the dryer 7 duringsuch an unsteady operation, may have moisture remaining in the compacts.If such wet compacts are rapidly heated in the reducing furnace 1 in asubsequent step, surface portions of the compacts may peel off, or thecompacts may rupture.

[0011] A heating gas in a dryer like the above-mentioned dryer 7, or ina dryer using hot air from a heat exchanger or the like as a heat sourcefor drying compacts (wet compacts), may cause compact rupture, orformation of a combustible gas from coal in the compacts, if thetemperature of the heating gas is high. To avoid these risks, themaximum temperature of the heating gas is set at 200° C. or lower.Depending on the moisture content, etc. of the heating gas, however, ahigher gas temperature than that may be set. Since a conventional dryeruses a heating gas whose temperature has been set to be somewhat low, ithas posed the problem of taking time for drying compacts.

[0012] In an apparatus for producing reduced iron, which uses coal as areducing agent, volatile matter (hereinafter referred to as VM), such asCO, CH₄, H₂O, CO₂ and N₂, occurs from coal, if the temperature of theheating gas is too high. At a high oxygen concentration, therefore, coalmay catch fire. Once VM develops in the dryer, the VM cannot be utilizedas a heat source in the reducing furnace in the subsequent step. Thisposes the disadvantage that the thermal efficiency of the reducingfurnace lowers. If the temperature of the heating gas is the sulfuricacid dew point (120° C.) or lower, on the other hand, corrosion will beinduced because of dew formation in piping, etc. inside the dryer.

[0013] As described above, temperature control for the heating gas is ofvital importance in efficiently drying compacts (wet compacts) in adryer. There has been an intense demand for the realization of a dryercapable of stable drying.

SUMMARY OF THE INVENTION

[0014] The present invention has been proposed in light of thesecircumstances. It is an object of this invention to provide an apparatusfor producing reduced iron, which can perform highly efficient, stabledrying in a dryer and produce high quality reduced iron stably; and alsoto provide a method for drying compacts which is applied to theapparatus.

[0015] A first aspect of the present invention, as a means of attainingthe above object, is a method for drying compacts, the method beingapplied to an apparatus for producing reduced iron by mixing andagglomerating a powder of a reducing agent and a powder of iron oxide ina pelletizer to form compacts or in a briquetter to form briquettes,drying the compacts (pellets, or briquettes) in a dryer, and reducingthe dried compacts in a high temperature atmosphere in a reducingfurnace, wherein

[0016] a temperature range of a heating gas supplied to the dryer is setbased on the following equation:

Sulfuric acid dew point≦T _(g)≦100/40·C _(H2O)+200

[0017] where T_(g) denotes the temperature [° C.] of the heating gas,and C_(H2O) denotes a moisture concentration [vol %] in the heating gas.

[0018] According to the above aspect of the invention, a high gastemperature adapted for the moisture concentration (moisture content) inthe heating gas can be set. Thus, the drying time can be shortened, andhighly efficient, stable drying can be performed, so that high qualityreduced iron can be produced stably. Furthermore, the temperature of theheating gas on the exit side of the dryer is a high temperature abovethe sulfuric acid dew point. Thus, acid corrosion of piping, etc.minimally occurs.

[0019] In the method for drying compacts as the first aspect of theinvention, the apparatus for producing reduced iron may use coal as thereducing agent, and the temperature T_(g) of the heating gas may be setat T_(g)≦300° C. Thus, compact rupture or formation of VM from coal inthe dryer can be prevented. Consequently, ignition of coal, or adecrease in the thermal efficiency in the reducing furnace in thesubsequent step can be prevented.

[0020] A second aspect of the invention is an apparatus for producingreduced iron, comprising a pelletizer or a briquetter for mixing andagglomerating a reducing agent and iron oxide to form compacts, a dryerfor drying the compacts, a reducing furnace for reducing the driedcompacts in a high temperature atmosphere, a first heat exchanger forperforming heat exchange between a hot off-gas discharged from thereducing furnace and combustion air to be supplied to the reducingfurnace, and a cooler for cooling the hot off-gas, wherein

[0021] a second heat exchanger for heating drying air is disposed on anexit side of the first heat exchanger, and the drying air heated by thesecond heat exchanger is supplied to the dryer.

[0022] According to this aspect of the invention, compacts (wetcompacts) are dried with moisture-poor drying air. Thus, sticking of thecompacts to each other does not take place (compacts are prevented frombecoming large lumps), and the compacts are uniformly dried. Since thecompacts are uniformly dried to leave no moisture behind inside thecompacts, peeling of the surface portion, or rupture of the compacts canbe avoided in the reducing furnace in the subsequent step. Moreover,high quality dry compacts are formed, and the supply of these compactsto the rotary hearth type reducing furnace can result in the stableproduction of high quality reduced iron.

[0023] In the apparatus for producing reduced iron as the second aspectof the invention, the cooler may be a water spray type first coolerprovided upstream from the first heat exchanger, an air introductiontype second cooler may be provided on a path bypassing the first cooler,and a control means may be provided for switching a valve provided at abifurcation upstream from the bypass path to select either the firstcooler or the second cooler based on a trade-off between a flow rate ofthe hot off-gas and a flow rate of the compacts to the dryer. Thus, inaddition to the same actions and effects as obtained by the secondaspect of the invention, the heat exchange efficiency of the first heatexchanger and the second heat exchanger is increased. Consequently, evenmore efficient, stable drying can be performed in the dryer.

[0024] In the apparatus for producing reduced iron as the second aspectof the invention, moreover, a hot stove for generating a hot gas may bedisposed on a drying air introduction side of the dryer, and the hot gasgenerated by the hot stove and the drying air heated by the second heatexchanger may be supplied to the dryer. Thus, in addition to the sameactions and effects as obtained by the second aspect of the invention,there is obtained the advantage that the temperature and flow rate ofthe drying air fed to the dryer can be adjusted more easily.

[0025] A third aspect of the present invention is an apparatus forproducing reduced iron by agglomerating a powder of a reducing agent anda powder of iron oxide in a pelletizer to form compacts or in abriquetter to form briquettes, drying the compacts in a dryer, andreducing the dried compacts in a high temperature atmosphere in areducing furnace, wherein

[0026] a hot stove for generating a hot gas is disposed on a drying gasintroduction side of the dryer, and the hot gas from the hot stove issupplied to the dryer as a drying gas.

[0027] According to this aspect of the invention, in addition to thesame actions and effects as obtained by the second aspect of theinvention, there is obtained the advantage that the operation of thedryer can be controlled easily by arbitrarily adjusting the temperatureand flow rate of the hot gas generated by the hot stove.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The above and other objects, features and advantages of thepresent invention will become more apparent from the followingdescription taken in connection with the accompanying drawings, inwhich:

[0029]FIG. 1(a) shows data indicating the conditions for and the resultsof compact (wet green compact, WGC) drying as a first embodiment of thepresent invention;

[0030]FIG. 1(b) is a graph showing the relation between the moistureconcentration in a heating gas and the temperature of the heating gas;

[0031]FIG. 2 is a schematic constitution drawing of an apparatus forproducing reduced iron, showing a second embodiment of the invention;

[0032]FIG. 3 is a schematic constitution drawing of an apparatus forproducing reduced iron, showing a third embodiment of the invention;

[0033]FIG. 4 is a schematic constitution drawing of an apparatus forproducing reduced iron, showing a fourth embodiment of the invention;

[0034]FIG. 5 is a schematic constitution drawing of an apparatus forproducing reduced iron, showing a fifth embodiment of the invention; and

[0035]FIG. 6 is a schematic constitution drawing of a conventionalapparatus for producing reduced iron.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Embodiments of the present invention will now be described indetail with reference to the accompanying drawings, which in no waylimit the invention.

[0037] [First Embodiment]

[0038]FIG. 1(a) shows data indicating the conditions for and the resultsof compact (wet green compact, WGC) drying as a first embodiment of thepresent invention. FIG. 1(b) is a graph showing the relation between themoisture concentration in a heating gas and the temperature of theheating gas.

[0039] As shown in FIGS. 1(a) and 1(b), the inventors conductedexperiments with an apparatus for producing reduced iron byagglomerating a powder of a reducing agent and a powder of iron oxide ina pelletizer to form compacts or in a briquetter to form briquettes,drying the compacts in a dryer, and reducing the dried compacts in ahigh temperature atmosphere in a reducing furnace. From the experiments,they found a method for setting the temperature range of a heating gasfor drying the compacts in the dryer as an equation dependent on themoisture concentration. The equation is offered below. In the column “VMgeneration” of FIG. 1(a), ◯ represents no generation of VM, and Xrepresents the generation of VM. In the column “Compact rupture”, ◯represents no compact rupture, and X represents compact rupture. In thecolumn “Evaluation”,  represents ◯ for both of VM generation andcompact rupture, while X represents X for one or both of VM generationand compact rupture. In FIGS. 1(a) and 1(b), the bulk flow velocity ofthe heating gas is 5.0 m/s or less.

Sulfuric acid dew point≦T_(g)≦100/40·C _(H2O)+200

[0040] where T_(g) denotes the temperature [° C.] of the heating gas,and C_(H2O) denotes a moisture concentration [vol %] in the heating gas.

[0041] According to the above equation, a high gas temperature adaptedfor the moisture concentration (moisture content) in the heating gas canbe set. Thus, the drying time can be shortened, and highly efficient,stable drying can be performed, so that high quality reduced iron can beproduced stably. Furthermore, the temperature of the heating gas on theexit side of the dryer is a high temperature above the sulfuric acid dewpoint. Thus, acid corrosion of piping, etc. minimally occurs.

[0042] In the apparatus for producing reduced iron, which uses coalcontaining VM as the reducing agent, the temperature T_(g) of theheating gas is set at T_(g)≦300° C. Thus, compact rupture or evaporationof VM from coal in the dryer can be prevented. Consequently, ignition ofcoal, or a decrease in the thermal efficiency in the reducing furnace inthe subsequent step can be prevented.

[0043] As described above, a high gas temperature adapted for themoisture concentration (moisture content) in the heating gas can be setaccording to the present embodiment. Therefore, wet compacts can beprevented from becoming large lumps, and peeling of the surface portionof the compacts, or rupture of the compacts in the reducing furnace canbe avoided, in an iron oxide reducing apparatus, as shown in FIG. 6,which cools a hot off-gas from the reducing furnace by a water spraytype primary cooler and a water spray type secondary cooler, and whichhas a dryer using the cooled hot off-gas as a heating gas. Needless tosay, the present embodiment can be applied to an iron oxide reducingapparatus equipped with a dryer using hot air from a heat exchanger orthe like as a heat source for drying compacts (wet compacts).

[0044] [Second Embodiment]

[0045]FIG. 2 is a schematic constitution drawing of an apparatus forproducing reduced iron, showing a second embodiment of the invention. InFIG. 2, the same members as in FIG. 6 explained in connection with theearlier technology are assigned the same reference numerals, andoverlapping explanations are omitted.

[0046] In the present embodiment, a heat exchanger 20 for heatingincoming air is provided. Drying air, which has been heated by the heatexchanger 20, is supplied to a dryer 7. This moisture-poor drying airdries compacts (wet compacts).

[0047] As shown in FIG. 2, the heat exchanger 20 of the invention (thesecond heat exchanger in the second aspect of the invention) is disposedon the exit side of a heat exchanger 9 (the first heat exchanger in thesecond aspect of the invention). This heat exchanger 20 heats incomingair, for example, by passing a constant amount of the incoming airthrough a pipe, and flowing a hot off-gas from the heat exchanger 9outside the pipe for heat exchange. This heated incoming air is suppliedto the dryer 7 as drying air.

[0048] On the exit side of the heat exchanger 20, a secondary cooler 10of a water spray type is disposed for cooling the hot off-gas dischargedfrom the heat exchanger 20. On the exit side of the secondary cooler 10,a dust collector 11 is disposed for cleaning the cooled hot off-gas.Other constitutions are the same as in FIG. 6 showing the earliertechnology, and their explanations are omitted.

[0049] According to the foregoing constitution, a powder of coal(reducing agent) and a powder of iron ore (iron oxide) as raw materialsare formed into compacts by a pelletizer or a briquetter 6, and dried bythe dryer 7. Then, the dried compacts (dry compacts) are fed to anintroducing device 2 of a reducing furnace 1.

[0050] The drying air fed to the dryer 7 is the incoming air heated bythe heat exchanger 20. This incoming air flows as an upflow through aninlet at a lower portion of the dryer 7, and dries the compacts (wetcompacts). Then, this drying air flows as a downflow, is discharged outof the system through an outlet, and dissipated into the air. Inside thedryer 7, the compacts (wet compacts) are dried with the drying air scantin moisture. Thus, the condensation and deposition of moisture on thecompacts are prevented. Consequently, sticking of the compacts to eachother does not take place, and the compacts are uniformly dried.

[0051] While a rotary hearth of the reducing furnace 1 is making nearlyone rotation, the dried compacts (dry compacts) fed onto the rotaryhearth undergo a reductive action by radiant heat due to combustioncaused by a burner 5 supplied with fuel and combustion air. The reducedcompacts P are discharged from a screw type discharger 4, delivered to aportable container 13 by a discharge chute 12, and transported to asubsequent step.

[0052] The hot off-gas is discharged from an off-gas duct 3, andprimarily cooled by a primary cooler 8 of a water spray type, and thenbrought to the heat exchanger 9, where the off-gas exchanges heat withthe combustion air. Further, the off-gas is carried to the heatexchanger 20 for heat exchange with incoming air, and secondarily cooledby the secondary cooler 10 of a water spray type. Then, the cooledoff-gas is passed through the dust collector 11, where it is cleaned,and then dissipated into the air.

[0053] According to the present embodiment, as described above, the heatexchanger 20 for heating incoming air is provided on the exit side ofthe heat exchanger 9. The heated incoming air is supplied to the dryer 7as drying air. Thus, the compacts (wet compacts) are dried with thedrying air scant in moisture. As a result, sticking of the compacts toeach other does not take place (growth of the compacts to large lumps isprevented), and the compacts are uniformly dried. Since the compacts areuniformly dried to leave no moisture behind inside the compacts, peelingof the surface portion, or rupture of the compacts can be avoided, inthe reducing furnace 1 in the subsequent step. Moreover, high qualitydry compacts are formed, and the supply of these compacts to the rotaryhearth type reducing furnace can result in the stable production of highquality reduced iron.

[0054] In the present embodiment as well, the temperature of the heatinggas (incoming air) in the dryer 7 may be controlled in accordance withthe moisture concentration (moisture content) of the heating gas by useof the equation indicated in the First Embodiment, thereby achieving afurther improvement in the drying efficiency.

[0055] [Third Embodiment]

[0056]FIG. 3 is a schematic constitution drawing of an apparatus forproducing reduced iron, showing a third embodiment of the invention. InFIG. 3, the same members as in FIG. 6 explained in connection with theearlier technology, and in FIG. 2 of the Second Embodiment are assignedthe same reference numerals, and overlapping explanations are omitted.

[0057] In the present embodiment, a bypass path 21 is provided forbypassing the primary cooler 8 in the Second Embodiment (the firstcooler in the aforementioned optional aspect of the invention), an airintroduction type cooler (the second cooler in the aforementionedoptional aspect of the invention) 22 is provided on the bypass path 21,and a controller (a control means) 24 is provided for switching a valve23 provided at a bifurcation upstream from the bypass path 21 to selecteither the primary cooler 8 or the cooler 22 based on a trade-offbetween the flow rate of the hot off-gas and the flow rate of thecompacts to a dryer 7. In more detail, the controller 24 receives adetection signal from a flow meter 25 which is interposed downstreamfrom an off-gas duct 3 to detect the flow rate of the hot off-gas, and adetection signal from a flow meter (not shown) which is provided in thedryer 7 to detect the flow rate of compacts (wet compacts) fed to thedryer 7.

[0058] According to the present embodiment, in addition to the sameactions and effects as obtained by the Second Embodiment, the heatexchange efficiency of a heat exchanger 9 and a heat exchanger 20 isincreased, for example, by passing the hot off-gas through the airintroduction type cooler 22, rather than passing the hot off-gas throughthe primary cooler 8, on an unsteady occasion immediately afterinitiation of operation of the apparatus. Consequently, even moreefficient, stable drying can be performed in the dryer 7. On thisoccasion, condensation of moisture on the wet compacts, followed bymutual sticking of the compacts and associated growth of the compacts tolarge lumps, can be prevented. Furthermore, peeling of the surfaceportion from the compacts, or rupture of the compacts can be avoided inthe reducing furnace 1 in the subsequent step.

[0059] In the present embodiment as well, the temperature of the heatinggas (incoming air) in the dryer 7 may be controlled in accordance withthe moisture concentration (moisture content) in the heating gas by useof the equation indicated in the First Embodiment, whereby a furtherimprovement in the drying efficiency can be achieved.

[0060] As a modification of the present embodiment, the primary cooler 8may be composed of a cooler having a water line and an air line, insteadof providing the bypass path 21. Either the water line or the air linemay be selected by the controller 24 based on a trade-off between theflow rate of the hot off-gas and the flow rate of compacts to the dryer.

[0061] [Fourth Embodiment]

[0062]FIG. 4 is a schematic constitution drawing of an apparatus forproducing reduced iron, showing a fourth embodiment of the invention. InFIG. 4, the same members as in FIG. 6 explained in connection with theearlier technology, and in FIG. 2 of the Second Embodiment are assignedthe same reference numerals, and overlapping explanations are omitted.

[0063] The present embodiment corresponds to the Second Embodiment ofFIG. 2, in which a hot stove 30 is additionally disposed on a drying airintroduction side of a dryer 7, and is connected to a drying air inletlocated at a lower portion of the dryer 7 so that a hot gas generated bythe hot stove 30 is supplied through the inlet together with dryingincoming air from a heat exchanger 20. Other constitutions are the sameas in the Second Embodiment.

[0064] The drying air fed to the dryer 7 is a combination of theincoming air heated by the heat exchanger 20, and the hot gas generatedby the hot stove 30 and supplied supplementally. This combined fluidflows as an upflow through the drying air inlet at the lower portion ofthe dryer 7, and dries the compacts (wet compacts). Then, this fluidflows as a downflow, is discharged out of the system through an outlet,and dissipated into the air. Inside the dryer 7, the wet compacts aredried with the drying air scant in moisture. Thus, the condensation ofmoisture on the wet compacts is prevented. Consequently, sticking of thewet compacts to each other does not take place, and the wet compacts areuniformly dried.

[0065] In response to changes in the temperature and flow rate of thedrying air fed from the heat exchanger 20, the temperature and flow rateof the hot gas fed from the hot stove 30 are adjusted, whereby theoperation of the dryer 7 can be controlled easily.

[0066] According to the present embodiment, as described above, the heatexchanger 20 for heating drying air, and the hot stove 30 for generatinga hot gas are provided concurrently to supply both the drying air andthe hot gas to the dryer 7. Thus, in addition to the same actions andeffects as obtained by the Second Embodiment, there is obtained theadvantage that the temperature and flow rate of the drying air fed tothe dryer 7 can be adjusted more easily.

[0067] [Fifth Embodiment]

[0068]FIG. 5 is a schematic constitution drawing of an apparatus forproducing reduced iron, showing a fifth embodiment of the invention. InFIG. 5, the same members as in FIG. 6 explained in connection with theearlier technology, and in FIG. 2 of the Second Embodiment are assignedthe same reference numerals, and overlapping explanations are omitted.

[0069] The present embodiment corresponds to the Second Embodiment ofFIG. 2, in which the heat exchanger 20 is abolished, and only a hotstove 30 is disposed on a drying gas introduction side of a dryer 7, andis connected to a drying gas inlet located at a lower portion of thedryer 7 so that a hot gas generated by the hot stove 30 is suppliedthrough the inlet. Other constitutions are the same as in the SecondEmbodiment.

[0070] A drying gas fed to the dryer 7 is the hot gas generated by thehot stove 30. The hot gas generated by the hot stove 30 f lows as anupflow through the drying gas inlet at the lower portion of the dryer 7,and dries wet compacts. Then, the hot gas flows as a downflow, isdischarged out of the system through an outlet, and dissipated into theair. Inside the dryer 7, the wet compacts are dried with the drying gasscant in moisture. Thus, the condensation of moisture on the wetcompacts is prevented. Consequently, sticking of the wet compacts toeach other does not take place, and the wet compacts are uniformlydried.

[0071] The foregoing constitution eliminates influences from thetemperature and flow rate of the hot off-gas from the reducing furnace1. The operation of the dryer 7 can be controlled easily by arbitrarilyadjusting the temperature and flow rate of the hot gas generated by thehot stove 30.

[0072] According to the present embodiment, as described above, the hotstove 30 for generating a hot gas is disposed, and the hot gas issupplied to the dryer 7 as a drying gas. Thus, in addition to the sameactions and effects as obtained by the Second Embodiment, there isobtained the advantage that the temperature and flow rate of the dryinggas fed to the dryer 7 can be adjusted more easily.

[0073] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for drying compacts, said method beingapplied to an apparatus for producing reduced iron by mixing andagglomerating a powder of a reducing agent and a powder of iron oxide ina pelletizer to form compacts or in a briquetter to form briquettes,drying the compacts in a dryer, and reducing the dried compacts in ahigh temperature atmosphere in a reducing furnace, wherein a temperaturerange of a heating gas supplied to the dryer is set based on thefollowing equation: Sulfuric acid dew point≦T _(g)≦100/40·C _(H20)+200where T_(g) denotes a temperature [° C. ] of the heating gas, andC_(H2O) denotes a moisture concentration [vol %] in the heating gas. 2.The method for drying compacts as claimed in claim 1, wherein theapparatus for producing reduced iron uses coal as the reducing agent,and the temperature T_(g) of the heating gas is set at T_(g)≦300° C. 3.An apparatus for producing reduced iron, comprising a pelletizer or abriquetter for mixing and agglomerating a reducing agent and iron oxideto form compacts, a dryer for drying the compacts, a reducing furnacefor reducing the dried compacts in a high temperature atmosphere, afirst heat exchanger for performing heat exchange between a hot off-gasdischarged from the reducing furnace and combustion air to be suppliedto the reducing furnace, and a cooler for cooling the hot off-gas,wherein a second heat exchanger for heating drying air is disposed on anexit side of the first heat exchanger, and the drying air heated by thesecond heat exchanger is supplied to the dryer.
 4. The apparatus forproducing reduced iron as claimed in claim 3, wherein the cooler is awater spray type first cooler provided upstream from the first heatexchanger, an air introduction type second cooler is provided on a pathbypassing the first cooler, and a control means is provided forswitching a valve provided at a bifurcation upstream from the bypasspath to select either the first cooler or the second cooler based on atrade-off between a flow rate of the hot off-gas and a flow rate of thecompacts to the dryer.
 5. The apparatus for producing reduced iron asclaimed in claim 3, wherein a hot stove for generating a hot gas isdisposed on a drying air introduction side of the dryer, and the hot gasgenerated by the hot stove and the drying air heated by the second heatexchanger are supplied to the dryer.
 6. An apparatus for producingreduced iron by agglomerating a powder of a reducing agent and a powderof iron oxide in a pelletizer to form compacts or in a briquetter toform briquettes, drying the compacts in a dryer, and reducing the driedcompacts in a high temperature atmosphere in a reducing furnace, whereina hot stove for generating a hot gas is disposed on a drying gasintroduction side of the dryer, and the hot gas from the hot stove issupplied to the dryer as a drying gas.